In a typical prior art predistortion system for the linearization of power amplifiers, the system typically contains multiple signal transmission paths, such as a reference path and a feedback path. In a predistortion linearization system, when a signal passes through different signal transmission paths, such as a reference path and a feedback path, the occurrence of a timing difference caused by the distinct signal paths is inevitable. This difference, usually referred to as a time delay, presents significant problems with respect to the accuracy of the predistortion correction. These problems are made worse by the fact that the time delay can vary with environmental conditions including temperature, system conditions including signal power level, system aging, and so on. Therefore, it is difficult to measure in a laboratory such an intrinsic time-delay parameter, and, moreover, a parameter for correcting the time-delay cannot be designed as a constant. Many efforts have been made in the prior art to compensate for, reduce or eliminate this time-delay. Traditionally, two methods have been used in prior art predistortion circuits to solve the problem created by the time-delay.
The first is to make a special time-delay cable by measuring and calculating the difference of the same signal passing through different transmission paths so as to compensate the time-delay effect, such as the processing in analog feed-forward predistortion systems. This approach suffers the limitation that the time-delay cable imposes a fixed time-delay that cannot be adjusted despite changes in the signal and environment which naturally occur during the operation of practical systems.
The second is to use a special digital signal processing (DSP) algorithm and circuitry to calculate and adjust adaptively the time difference of the same signal passing through different transmission paths, and then use the resulting time-delay information to correct the non-linearity of power amplifier. This approach is usually implemented in a digital feedback approach and a wireless environment. However, an extra circuit, typically a latch, and an associated algorithm are needed, and the accuracy of the time-delay calculation is also related to the convergence rate of the algorithm.